The present invention regards an electronic ignition device with limitation of the voltage at an ignition coil primary winding terminal.
As is known, one of the problems present in electronic ignition devices for inductive loads is to limit the voltage at the primary winding terminal of the ignition coil, in the event of a malfunctioning of the device being detected, so as to prevent an ignition spark from being generated on the secondary winding terminal of the same coil.
In this connection, FIG. 1 shows a schematic circuit diagram of an electronic ignition device 1 comprising an ignition coil 2 and a power element 3, for example an IGBT or a bipolar power transistor. In greater detail, the ignition coil 2 includes a primary winding 2a and a secondary winding 2b; a first terminal 2c of the primary and secondary windings 2a, 2b is connected to a supply line 4, set at a battery voltage VB, a second terminal 5 of the primary winding 2a is connected to a collector terminal of the power element 3, and a second terminal 6 of the secondary winding 2b is connected to a spark plug (not shown in FIG. 1) which generates the ignition spark. The power element 3 has an emitter terminal 7 connected to ground GND and a control terminal 8 connected to a microprocessor 9, shown only schematically in FIG. 1, through a resistor 10. A high voltage Zener diode 25 has its cathode connected to the second terminal 5 of the primary winding 2a and its anode connected to the control terminal 8 of the power element 3. The high voltage Zener diode 25 limits the maximum voltage applied to the second terminal 5 of the primary winding 2a to prevent the latter from exceeding the breakdown voltage of the power device 1.
The microprocessor 9 controls turning on of the power element 3 by supplying, to the control terminal 8 of the latter, a trigger signal at a high logic level (FIG. 2). Upon turning on of the power element 3, across the primary winding 2a a voltage is applied that is close to the battery voltage VB. Consequently, a primary current Iout starts flowing in the terminal of the primary winding 5 (FIG. 2).
Once an appropriate time has elapsed during which the primary current Iout reaches a preset value Io (charging time of the ignition coil 2), the microprocessor 9 controls turning off of the power device 3 by sending the trigger signal to a low logic level. In this condition, a voltage pulse V0, is generated at the second terminal 5 of the primary winding 2a (FIG. 2); the voltage pulse, transferred onto the second terminal of the secondary winding 2b multiplied by the turn ratio of the ignition coil 2, gives rise to a spark.
In an electronic ignition device of the type described above, it is necessary that the spark is generated only when the microprocessor 9 turns off the power element 3 by sending the trigger signal to the low logic level. However, in the event of malfunctioning of the device, it may be necessary to turn off the power element 3 independently of the logic level of the trigger signal and without a spark being produced on the second terminal 6 of the secondary winding 2b. 
As shown in FIG. 3, to meet this requirement, the electronic ignition device 1 is provided with a protection circuit 11, shown only schematically in FIG. 3, for detecting anomalous operating conditions of the electronic ignition device 1, such as overheating of the power element 3 or exceeding the preset current value Io, and supplying, at an output terminal 16, a logic signal EN used as enable signal for a voltage limiting circuit 12.
In greater detail, the voltage limiting circuit 12 has a first input terminal 13, a second input terminal 14, and an output terminal 15. The first input terminal 13 of the voltage limiting circuit 12 is connected to the second terminal 5 of the primary winding 2a; the second input terminal 14 of the voltage limiting circuit 12 is connected to the output terminal 16 of the protection circuit 11 through an inverter 17; and the output terminal 15 of the voltage limiting circuit 12 is connected to the control terminal 8 of the power element 3.
The voltage limiting circuit 12 comprises an enable transistor 18 of the NPN type, having a collector terminal connected to the first input terminal 13 of the voltage limiting circuit 12 through a high voltage resistor 19, an emitter terminal connected to ground GND, and a control terminal connected to the second input terminal 14 of the voltage limiting circuit 12.
The voltage limiting circuit 12 further comprises a first high voltage vertical transistor 20a and a second high voltage vertical transistor 20b , both of the NPN type and coupled in Darlington configuration. In particular, the first high voltage vertical transistor 20a has a collector terminal connected to the first input terminal 13 of the voltage limiting circuit 12, a control terminal connected to the collector terminal of the enable transistor 18 through a first circuit node 30, and an emitter terminal. The second high voltage vertical transistor 20b has a collector terminal connected to the first input terminal 13 of the voltage limiting circuit 12, a control terminal connected to the emitter terminal of the first transistor 20a , and an emitter terminal connected to the output terminal 15 of the voltage limiting circuit 12 through a Zener diode 22. The Zener diode 22 has its cathode connected to the emitter terminal of the second transistor 20b and its anode connected to the output terminal 15 of the voltage limiting circuit 12. A resistive element 21 is connected between the control terminal and the emitter terminal of the second high voltage vertical transistor 20b. 
The electronic ignition device 1 further comprises a protection transistor 23 having a collector terminal connected to the control terminal 8 of the power element 3 via a second circuit node 31, an emitter terminal connected to ground GND, and a control terminal connected to the output terminal 16 of the protection circuit 11.
A biasing resistor 24 is coupled between the second circuit node 31 and the output terminal 15 of the voltage limiting circuit 12.
After detecting a malfunctioning of the electronic ignition device 1, the protection circuit 11 generates, on the control terminal of the protection transistor 23, a high logic level of the logic signal EN. Consequently, the protection transistor 23 saturates, generating on the second circuit node 31 a voltage Vcesat equal to its own saturation voltage (voltage present between the collector and the emitter terminal of the protection transistor 23 in saturation) and determining turning off of the power element 3, with consequent increase in the voltage on the second terminal 5 of the primary winding 2a. 
At the same time, the inverter 17 generates, on the control terminal of the enable transistor 18, a logic signal, correlated to the logic signal EN, at a low logic level. Consequently, the enable transistor 18 turns off, generating on the first circuit node 30 a voltage that turns on the high voltage vertical transistors 20a and 2b. These transistors supply the Zener diode 22 and the biasing resistor 24 with a current that causes a biasing voltage VP across the biasing resistor 24. The biasing voltage VP causes turning on again of the power element 3, which maintains the voltage on the second terminal 5 of the primary winding 2a at a value VL that maintain the high voltage vertical transistors 20a , 20b on, so that the latter continue to supply current until complete exhaustion of the energy stored in the primary winding 2a of the ignition coil 2. In particular, the value VL is
VL=VR+Vbe1+Vbe2+VZ+VP+Vceatxe2x80x83xe2x80x83(1)
wherein VR is the voltage across the high voltage resistor 19, Vbe1 and Vbe2 are the voltages between the control and the emitter terminals of the high voltage vertical transistors 20a, 20b , and Vz is the voltage across the Zener diode 22.
At the end of the discharge of the ignition coil, the voltage VL reaches the value of battery voltage VB. In these conditions, for proper operation of the voltage limiting circuit 12, the high voltage vertical transistors 20a , 20b must be off. This occurs only if the voltage Vz satisfies the following condition:
VZ greater than VB VR Vbe1 Vbe2 VP Vcesatxe2x80x83xe2x80x83(2)
which is obtained from relation (1) setting VL=VB.
Consequently, on the basis of relation (2), in order to have proper operation of the voltage limiting circuit 12, the Zener diode 22 must be chosen each time according to the maximum battery voltage VB envisaged in the specifications.
In addition, in applications for electronic ignition that require a preset maximum value of 24 V for the battery voltage VB, the value of the voltage VL may be too high if the coil has a high turn ratio between its primary and its secondary windings; consequently, an undesired spark may be generated.
According to the principles of the invention, an electronic ignition device includes an ignition coil having a primary winding terminal and a secondary winding terminal generating a spark. A power element is arranged between the primary winding terminal and ground. A protection circuit issues a disable signal to the control terminal of the power element under preset conditions. A voltage limiting circuit having inputs connected to the primary winding terminal and to the battery voltage, and an output connected to the control terminal of the power element is provided. The voltage limiting circuit detects a potential difference between its own inputs, and supplies to the control terminal an activation signal for the power element, in presence of the deactivation signal and when the potential difference exceeds the supply voltage by a preset value. Thereby, the voltage limiting circuit limits the voltage on the primary winding terminal to a preset value, which depends upon the value of the battery voltage.